Pedantic is being the friendly, correcting cynic. Native Spanish speakers use si as si in texts and informal messages all the time. It's understood from context; I think.
It's not hugely different from using i instead of I as the first-person singular pronoun in English which I wouldn't class as 'abhorrent' but rather 'very informal', which is fine so long as the actual language itself is preserved.
In the case in question, a programming language, we can assume a context-free grammar, and therefore the keyword equivalent for "if" would be "si" and the keyword for "yes" would be "sí".
